18 BOTANICAL SURVEY OP THE C(EUR D'ALENE MOUNTAINS. 



which lie to the east of the St. Joseph and which connect with the 

 South Fork ridges at Wiessner Teak. Another deflection toward 

 the Coaur d'Alene Piver occurs here. Now the ridges to the south of 

 this valley and of its tributary the South Fork are much higher than 

 they are to the northward. The consequence is that a very large pro- 

 portion of the clouds which have accumulated here are driven north- 

 easterly into the basin of the North Fork. This probably explains 

 why the last-named area receives a larger precipitation than the other 

 parts of the Cceur d'Alene. 



The second line of deflection is near the northern part of the Cceur 

 d'Alene triangle. The wind, blowing from the southwest across the 

 plains of the Columbia, strikes with unbroken force the high ridges 

 which form the northern half of the western mountain rim of the 

 Cceur d'Aleues. As before, it is deflected toward the north. Its 

 course is then by the south end of Lake Pend Oreille, over this lake, 

 and eastward by the valley of the Clark Fork of the Columbia, As it 

 approaches the south end of Lake Pend Oreille the rapidly moving 

 column of* air, many kilometers wide, is compressed between the Cceur 

 d'Alenes and the mountain range to the west of the lake into a space 

 which has a width of less than 5 kilometers (3* miles). Passing 

 through this narrow gap, it strikes the lake with terrific force, the wind 

 sometimes reaching a measured velocity of 145 kilometers (90 miles) 

 an hour. In their course up the valley of the Clark Fork the storm 

 clouds deposit most of their burden of snow and rain in the lower por- 

 tion. The reason for this lies, as in other similar cases, in the configura- 

 tion of the mountains. The ridges bordering this valley on the north 

 have an average crest line of 1,900 meters (6,200 feet) altitude for a dis- 

 tance of about 130 kilometers (81 miles) east from the lake. This keeps 

 a far larger quantity of clouds confined in the valley than would be 

 the case were these mountains of a lower elevation. Above Thompson 

 the pent-up clouds begin to escape and spread out northeasterly by 

 way of the valley of the Thompson Piver. Twenty kilometers (12 

 miles) farther up the Clark Fork valley, at Horse Plains, the northern 

 ridges break away and permit a still further thinning out of the clouds. 

 We have, therefore, in the portion of the valley of the Clark Fork 

 between Thompson and Lake Pend Oreille, a region of excessive pre- 

 cipitation, locally known as the "Snow Belt. 7 ' Prior to the forest 

 fires of later years this carried an exceedingly dense forest growth. 

 Above Thompson the climate of the valley becomes one of very decided 

 aridity, resembling in many features that which prevails east of the 

 Eockies in this latitude, and having a flora which includes many of the 

 species of that region. 



The third area of deflection is in the upper St. Mary valley. The 

 ridges which form the divides between some of the western tributaries 

 of this stream and those which flow southward into the Clearwater are 

 comparatively low, their mean elevation being about 1,250 meters 

 (4,100 feet). 



